Heat transfer plates included in a plate heat exchanger each have an inlet and an outlet, and a wavy portion provided between the inlet and the outlet and waving in a direction in which the heat transfer plates are stacked. In such a plate heat exchanger, top parts of a wavy portion provided in one heat transfer plate that is on the lower side and bottom parts of a wavy portion provided in another heat transfer plate that is on the upper side overlap each other when seen in the stacking direction, forming overlapping parts, and are bonded to each other at the overlapping parts by brazing.
If waves of the wavy portion provided in each of the heat transfer plates do not have a uniform height, gaps may be provided between adjacent ones of the heat transfer plates even at the overlapping parts, that is, non-bonded parts where the heat transfer plates are not bonded to each other may occur. In general, a wavy portion of a heat transfer plate is formed by presswork. One of waves in the wavy portion that is provided adjacent to each of an inlet and an outlet (hereinafter referred to as “the first wave”) is positioned far from a crank shaft of a press machine and is therefore likely to have an error in wave height. Hence, the first wave tends to have a non-bonded part and to have low bonding strength.
Furthermore, a region near each of the inlet and the outlet is a planar surface not having the wavy portion, and the area thereof that is subject to pressure is large. Therefore, the stress working on a bonded part of the first wave that is provided adjacent to each of the inlet and the outlet is larger than the stress working on a heat transfer surface area in which the wavy portion is provided. Hence, the overlapping part of the first wave that is provided adjacent to each of the inlet and the outlet particularly needs to have high bonding strength.
Patent Literature 1 discloses a plate heat exchanger including walls provided around an inlet and an outlet. Patent Literature 2 discloses a plate heat exchanger including walls (reinforcing grooves) provided on a heat transfer surface area.